Alexa and Chroma du Monde

Alexa and Chroma du Monde

This isn't really a presentation of test shots like I usually do for this site. I just had a quick minute during a checkout to grab a little video I've been wondering about. Being involved in the engineering side of things, one of the points I'm most curious about the Arri Alexa is its colorimetry or how it arranges the colors it sees into video color space. I had my very handy RED CDM with me, along with Leader 5330 so I took a minute and had a look at the the two primary video output modes on the camera - Rec709 and Log C. 

Rec709 is the linear, HD video output of the camera and is the fastest, easiest thing to record on-board to SxS cards if you want to start working with the footage immediately and don't have time and/or money to do a big grade. The image in this mode is beautifully crisp and colorful with fairly pleasing skin tones. Seeing as ProRes 4444 HQ can be recorded to card in real time, the 4:4:4 color space has a lot of wiggle room for correction and grading in post production. However, since the color temperature and gamma is baked in, getting a correct white balance in the field is good idea. Use a gray card.

Here's a crappy Vimeo video of the shot. I personally can't wait for a more faithful web vid compression scheme. I make the highest quality H.264 video I can in Compressor and then after Vimeo transcodes to its format, my shots always look like poop. I wish I knew a better way. 

Here are the shooting specs:

Color Space: Extended

Gamma: Rec709

Captured to internal SxS recorder @ ProRes 4444 12 bit

800 ISO

3000 kelvin color correction -3

100mm Master Prime T8

Arri Alexa - Rec709 - Chroma du Monde from Ben Cain / Negative Spaces on Vimeo.

Vector Data @ 75% Saturation, Gain x1

And for context - Vector Data @ 75% Saturation, Gain x2

As you can see, Arri has done a good job of making a faithful color gamut that looks natural and pleasing without much user interference. If you've ever looked at Sony or Panasonic colors on the scope with their skewed factory color matrix, you'll know what a big deal it is to have nice colors right out of the gate.

The color space was set to Extended as opposed to Legal and I'm wondering if that's the reason there is some unfortunate compression artifacing in the super saturated colors seen on the chart, most noticeably in blue. This is 4444 HQ 12 bit though so it should be able to handle it. I'm a little surprised to be honest. I'll have a look next time in the Legal color space as well. 

So how much can you really tell from a Chroma du Monde chart? Not that much really! You need to a diverse set of scenes with varying degrees of contrast and color temperature to see what a camera can really deliver. The Chroma du Monde does however, represent a decent amount of natural looking colors you're bound to encounter so it can give you a general idea of what you've got to work with. 

If you really want to evaluate the quality of these ProRes files and how far you can push them in post, you need to have a look at the source vid so here are clips of all the shots from this post for you to download and peruse. 

negativespaces.squarespace.com/storage/video/Alexa_qt.zip

Here's the same thing in Log C.

Color Space: Extended

Gamma: Log C

Captured to internal SxS recorder @ ProRes 4444 12 bit

800 ISO

3000 kelvin color correction -3

100mm Master Prime T8

Arri Alexa - Log C - Chroma du Monde from Ben Cain / Negative Spaces on Vimeo.

I exposed for Rec709, switched it to Log C, and left the T Stop the same. I thought it looked good on the waveform but Arri actually recommends you set middle gray to 39% for optimum exposure in this mode. By these specs, my exposure may have been a little hot for Log C believe it or not. 

Compare the above waveform to the same for Rec709 below to get a sense of how compressed Log C is. 

Here is vector information for Log C - Saturation 75%, Gain x2:

Log C - Saturation 75%, Gain x1:

You can see how desaturated it is. It's also a totally different colorimetry than Rec709. The skin tone is pretty much in the same zone but Yellow, Green, and Cyan are markedly more subdued than Red, Magenta, and Blue.

As it's designed for the Digital Intermediate (DI), shooting in Log C will always buy you increased dynamic range from the camera. However it's far more work in post production as a grade/color correct will be required at some stage to linearize the image for final video delivery. Additionally, if editorial wants to start working with the Log material immediately and doesn't want to look at this flat, compressed state; the least that's required will be a display LUT applied to their monitors. 

Gary Adcock wrote this great article explaining the difference between Linear vs. Log and its origins. His explanation is far more knowledgeable and articulate so I definitely recommend giving it a read. 

The most bang for your latitude buck would be to shoot the Alexa in Log C ArriRAW, and record in T Link 444 to a data recorder like the S.Two OB-1. This will open up uncompressed, 2k Raw, 12 Bit Bayer data right off the sensor. How much of an advantage is this over shooting Log C in ProRes 4444 HQ right to card? That remains to be seen. This edit ready media is pretty powerful stuff and to be able to capture that kind of quality right in the camera is huge. 

From the Alexa Site:

http://www.arridigital.com/alexa/faq#8-2

Can the full exposure latitude of ALEXA be recorded as ProRes onto SxS cards or can the full range only be recorded using the HD-SDI or ARRIRAW outputs?

The short answer is: yes, the full exposure latitude of the ALEXA can be recorded as ProRes onto SxS cards. The long answer is this: The ALEXA sensor can capture 14 stops of exposure latitude. This image data is processed internally in a 16 bit format. Whenever this data is converted to an output, be it the 12 bit ARRIRAW, 10 bit HD-SDI, 12 bit ProRes 4444 or 10 bit ProRes 422 formats, the whole exposure range of the captured image is mapped from the 16 bit range into the respective output range (see graphic). While the range of brightest to darkest image content remains the same, the number of different lightness levels in between differ. A 10 bit image has fewer steps between the brightest and the darkest parts than a 12 bit image.

Arri Alexa - Log C - Chroma du Monde - 4 stops overexposed from Ben Cain / Negative Spaces on Vimeo.

Arri Alexa - Log C - Chroma du Monde - 3 stops underexposed from Ben Cain / Negative Spaces on Vimeo.

Now download those files:

negativespaces.squarespace.com/storage/video/Alexa_qt.zip

Canon Picture Styles and Chroma Du Monde

Canon Picture Styles and Chroma Du Monde

I was curious about the real differences on the video level between the various Canon HDSLR Picture Styles so I set up the DSC Labs RED CamBook Chroma Du Monde 28R and evenly lit it with ambient daylight.

Converting the camera's HDMI OUT to SDI, I set the exposure in the recommended way for the chart, putting the grayscale's crossing point at just under 60% on the waveform. I then made no exposure changes and just dialed through the various Canon Picture Styles capturing the waveform and vectorscope data from the Leader monitor.

Camera and Lens: Canon 5D Mark 2, Canon 24-105mm f/4 Zoom

All styles were adjusted to the following "standard practice" specs:

Contrast: all the way down

Sharpness: all the way down

Saturation: down 2 points

For Neutral and Faithful, the two "out of the box" Picture Styles that I find to be well suited for video, I looked at Saturation -1 as well to see how much difference 1 point makes on the scopes. The answer is a lot. Most people would agree that the colors on these cameras are over saturated and need to be backed off a bit to look more natural and less video. When adjusting a video camera's colorimetry using DSC charts, the theoretical goal is to ensure faithful color reproduction by aligning the primary colors into their targets. The color response that this creates however may not be suitable for all projects it may even look a bit over saturated compared to the low sat "film like" color matrices found in many prosumer camcorders.

Have a look:

Let's have a closer look at Faithful, Saturation -1

This is about as close as we can get to hitting our targets with the Canon 5D Mark 2. In my opinion this setting doesn't look as nice as Saturation -2 so as always, your eye is really your best tool for image evaluation.

Canon Picture Styles: Looking at these thumbnails alone, it's a little hard to tell the difference.

Waveforms: Glancing at these however you can see that there is a small difference in gamma response from Style to Style, Neutral being the most compressed in comparison.

Vectorscope: Even with the saturation turned down, Portrait and Landscape are very extreme color looks compared to the more muted tones of Neutral and Standard. Faithful on the other hand, does what it say it does and offers the most accurately aligned video colors of the bunch.

In order to really see the differences though, you need to look at the chart in the context of video. Here's a file for you, feel free to download it and take into FCP where you can open up the scopes and really scrutinize the differences between Picture Styles.

Note: RED CamBook has a highly reflective surface so must be angled back to avoid seeing yourself in it which is why the image is skewed in this video. . There is a small amount of gamma lift that happens in Vimeo upon conversion to their format. Why does it do that? I don't know and I'm still searching for a workaround.

Canon 5D Mark 2 Picture Styles and Chroma Du Monde from Ben Cain / Negative Spaces on Vimeo.

As for the Canon 7D and 1D Mark IV? From what I discovered from the tests Jem and I did at Sekonic, the color response from the various Picture Styles is very similar to the results found here.

DSC Labs' RED CamBook

DSC Labs' RED CamBook

I recently obtained one of these new RED CamBooks and in my opinion, it's one of DSC Labs' most useful products to date.

http://dsclabs-us.intelex.ca/product.php?product_id=143

While this product does bear RED's name, seeing as how there's no way to adjust RED's colorimetry in the camera other than some basic phase, saturation, and white balance controls, I fail to see why this is marketed as a "RED specific" tool. Yeah you can shoot the chart and use it to make picture adjustments after the fact but because it's a full 28 swatch Chroma Du Monde chart that fits it your laptop bag, it's also incredibly useful for painting and matching traditional HD video cameras. The other small cambooks DSC had sold prior to this don't have the full range of video colors as found on the full size CDM 28 chart.

This small "to go" Chroma Du Monde is exactly the same as the full size version in terms of video colors and tonality. Because of the size it can easily travel and if you find yourself matching cameras in a hotel room in the middle of nowhere, you can now do so with a far greater degree of precision than with the older, 6 or 12 video color CamBooks.

A new feature to the chart is the addition of 6 super saturated, non-calibrated primary colors on the outside of the CDM. This is apparently a feature RED requested as the Chroma Du Monde colors are low saturation and intended to be normal, life-like colors (hence the name, "Color of the World"). The 6 non-calibrated colors are intended to be used as a visual reference so you can quickly glance at the monitor and confirm that the camera is still seeing high-sat even if you've calibrated to the low-sat colors. It's an interesting feature. I personally haven't found myself using it but you never know.

I recently used the RED CamBook to have a look at the new RED Mysterium-X sensor across the entire range of camera ISO's and made some very interesting discoveries. Until next time..

Update

on 2010-07-13 04:35 by Ben Cain

Nice to see there's quite a bit of online activity at the moment regarding the RED CamBook. Maybe I wasn't clear enough in my original post but yes, I do realize how this product was intended to be used with the RED and that is to align the camera image to the chart or multiple cameras to the chart in post production. With a camera like this, the emphasis is always on what can be done with the images in post production though with the new RedColor color science and RedGamma, you can now get much closer to your preferred look right in the camera. However like I mentioned, because of it what it is, a full swatch Chroma Du Monde, it's also very handy for use with more "conventional" Rec709 video cameras.